Heavy duty vehicles such as tractors and other work machines, can be subject to widely varying loads. When a relatively heavy load is applied to the rear end of the vehicle, such as when an implement or tool pulled through the ground by a tractor encounters soil conditions resulting in much increased drag, resulting in application of an increased downward force on rear wheels of the tractor, the front end of the tractor can be lifted as a result. If the tractor or other vehicle has a suspended axle, that is, the front end of the frame or body of the tractor or vehicle is connected to the front axle by a suspension, the front end can be lifted relative to the suspended axle, thereby increasing a distance therebetween. An effect of this is that the downward force applied against the ground by the front wheels can be decreased. This can be a particular problem when the front wheels are driving or powered, as the reduction of downward force will result in loss of front wheel traction.
Many tractors and other vehicles are known to have automatic load leveling and/or traction control systems which employ one or more hydraulic cylinders disposed between the suspended front axle and the frame of the vehicle. If the tractor or other vehicle has mechanical front drive (MFD), engagement of the MFD can cause the front wheels to generate traction force for pushing the vehicle forward. In operation, the front wheel traction force of MFD can produce a moment acting against the suspension system to swing it outward relative to a frame of the vehicle, and thereby cause the suspension cylinders to extend and the vehicle front end to rise. In a traction control mode, the cylinders are automatically extended by the system as required to ensure that a desired or required vehicle leveling and/or front traction is maintained. As a result, presence of a signal activating the MFD can serve to indicate the tendency of cylinder extension due to front traction. Additionally, the MFD signal can indicate possibility of dynamic load transfer conditions under heavy load that will tend to extend the cylinders.
Also, many such systems include fluid lines connecting the rod ends and piston ends of the cylinders in a typical regenerative circuit, which allow free fluid flow between the ends of the cylinders, for instance, when loading conditions which tend to urge the front end upwardly are encountered, such as when an implement or tool is being pulled through soil or other material resulting in high drag forces. This can be a problem, as the cylinders may fully extend, substantially reducing or eliminating the capability to extend more, and making retraction actions for absorbing shock loads less cushioned, resulting in a rougher ride. The system may also allow the cylinders to extend and increase the distance between the front end and the front axle, such that, as a result, the front end is raised, and the implement or tool is urged deeper into the ground, resulting in greater drag and increased forces urging the front end upwardly, which can cause still further extension of the cylinders in an attempt to increase or maintain front traction. The system can thus be caught in an undesirable loop, such that suspension performance is degraded and field work quality can deteriorate.
Thus, what is sought is an improved system and/or method of operation of a control for a suspended front axle which overcomes one or more of the problems and disadvantages set forth above.